This invention relates to the use of molecular oxygen for the coupling of aromatic compounds including substituted aromatics such as phenols or alkyl benzenes to form polyaromatic compounds such as phenylphenol, dimethylbiphenyl, dicarboxylic acid biphenyl and others.
Coupled aromatic compounds and particularly substituted coupled aromatic compounds have been found to be useful, environmentally safe functional fluids. Disubstituted polyaromatic compounds are also suitable as polymer intermediates. Polymers formed from such monomers will have aromatic backbones and have properties normally associated with such polymers such as high temperature resistance and flame retardancy.
It has been customary to couple benzene to form diphenyl by passing benzene through a hot tube heated to 650.degree. to 850.degree. C. Such a reaction is not suitable for producing substituted diphenyls from substituted benzenes. Such severe conditions usually attack the substituent causing an unwanted loss of reactant and reaction product. When substituted biphenyls are desired, one method frequently employed is the halogenation of the substituted benzene followed by the reaction of that compound at an elevated temperature around 300.degree. C in the presence of copper powder. The reaction produces a low yield, however, and is additionally expensive due to the large amount of copper powder converted to the halide. The above reaction is furthermore not generally applicable to substituted aromatics since the severity of the reaction conditions precludes the use of many substituted aromatic compounds.
It has been known that certain metal compounds can act as oxidizing agents and can be used to couple aromatics under somewhat milder conditions. These certain metals in high oxidation states can be used to form diaromatics while they are simultaneously reduced to a lower valence state. Diphenyl mercury has been shown to form biphenyl and metallic mercury and palladium acetate has been shown to form diaromatics and metallic palladium. Because expensive metallic salts are used stoichiometrically in these reactions they are of little practical value. Until now it has not been possible to use inexpensive oxidizing agents such as molecular oxygen to effect this coupling.
It has more recently been known that coupling of aromatics may be made catalytic with respect to palladium through the use of acyl mercury complexes; however, the ac 1 mercury salt is used in stoichiometric quantities. The coupling reaction requires a stoichiometric amount of mercuric salt to be reduced for each molecule of coupled product formed. An additional palladium catalytic coupling reaction couples the aromatics in the presence of a strong acid which reduces the utility of the reaction. Aromatics with substituents sensitive to acid, for example, toluene produces a major product as a result of side chain oxidation.
Attempts to use standard methods to reoxidize the expensive oxidizing agents previously known in the art have resulted only in methods which are prohibitively expensive or in which conditions were so severe that poor yields resulted.
I have now found that these problems can be circumvented through the use of a catalyst comprised of a Group VIII metal or metal oxyanion compound and a mercuric oxyanion compound in the presence of molecular oxygen at pressures of at least about 200 psi. Only catalytic amounts of the Group VIII metal or metal oxyanion compound and the mercuric oxyanion compound are utilized with the molecular oxygen to provide a process for coupling monoaromatic compounds.